In the operation of electric power plants, steam turbines typically have multiple throttle valves and/or governor valves to control the flow of steam to the turbine. The throttle valves, when provided, are utilized in starting up the turbine until it reaches approximately eighty-percent of synchronous speed, at which point control is transferred to the governor valves. In nuclear installations, the steam turbines are controlled solely by governor valves. Once the governer valves are controlling the admission of steam to the turbine, they may be controlled in the full arc or single valve mode of operation; i.e., where all the valves are operated to admit the same portion of the total steam flow through a full arc of nozzles; or they may be operated in the partial arc or sequential mode, where the steam is admitted through a partial arc by the operation of the valves in a predetermined sequence; i.e., only one or a single group of valves operate to control any variation in total steam flow to the turbine. The valves are operated usually in the single valve mode to heat up the turbine parts uniformly, and are operated in the sequential valve mode to minimize throttling losses due to a plurality of partially open valves and to more precisely control steam flow variations. In the event that one or more of the valves that control steam flow in the sequential mode should fail to respond properly to its control signal, the turbine consequently fails to respond to the proper speed, or load command. Further, the failure of a valve to respond in the sequential mode may cause the turbine to suffer what is known as "double shock," which occurs when there is discontinuity of steam admission through adjacent nozzle arcs. In other words, one arc of nozzles is admitting steam, the adjacent arc is admitting little or no steam, and the next adjacent arc is admitting steam. In such event, the turbine should be transferred from sequential valve operation to single valve operation in order to prevent damage to the turbine blades.
U.S. pat. application Ser. No. 404,057, entitled "System and Method for Operating a Steam Turbine with Protection Provisions for Valve Positioning Contingency," filed by Braytenbah and Podolsky on Oct. 5, 1973, and assigned to the common assignee discloses a digital computer controlled turbine power plant control system that includes a system for detecting the failure of a throttle or governor valve to respond to its respective control signal. In accordance with the teaching of such disclosure, the turbine control system includes a speed/load control function for generating desired valve position signals. It also includes provisions for generating signals which were representative of the actual valve positions. Each one of the actual valve position signals is compared with its respective desired valve position signal, and a contingency condition is detected to provide a suitable indication in the event the actual valve position does not agree with its desired valve position signal.
This system proved reliable in performing its required function. However, in certain circumstances, where a fast action of the valves is effected, particularly where a large change in valve position is required, a valve contingency would be detected while the position of the valve caught up with its control signal. Although this situation did not affect the reliability of the system in detecting an actual contingency, it necessitated precise adjustment of tolerances to prevent the detection of contingencies which could be considered as normal operation. In order to overcome the above condition, it was further proposed to provide a system where the relationship of the positioning or control signal and the actual valve position is compared to a given reference. Such a system, however, was unable to detect all conditions of failure.
In view of the above, it is desirable to provide a system having the reliability of the contingency system described in the copending application Ser. No. 404,057, yet does not detect any normal mismatch of the actual valve position and the valve positioning signal. Such a system then could provide for automatic transfer from sequential to single valve operation without inadvertent transfers occasioned by the described circumstances or lack of proper precise adjustment.